| What is a “gotcha”?The word “gotcha” started out as the expression “Got you!” This is something that someone who speaks idiomatic American English might say when he succeeds in playing a trick or prank on someone else. “I really got you with that trick!”
The expression “Got you!” is pronounced “Got ya!” or “Got cha!”. Among computer programmers, a “gotcha” has become a term for a feature of a programming language that is likely to play tricks on you to display behavior that is different than what you expect. Just as a fly or a mosquito can “bite” you, we say that a gotcha can “bite” you. |
About this Page
This is a page devoted to Python “gotchas”. Python is a very clean and intuitive language, so it hasn’t got many gotchas, but it still has a few that often bite beginning Python programmers. My hope is that if you are warned in advance about these gotchas, you won’t be bit quite so hard!
Note that a gotcha isn’t necessarily a problem in the language itself. Rather, it is a situation in which there is a mismatch between the programmer’s expections of how the language will work, and the way the language actually does work. Often, the source of a gotcha lies not in the language, but in the programmer. Part of what creates a programmer’s expectations is his own personal background. A programmer with a Windows or mainframe background, or a background in COBOL or the Algol-based family of languages (PL/1, Pascal, etc.), is especially prone to experiencing gotchas in Python, a language that evolved in a Unix environment and incorporates a number of conventions of the C family of programming languages (C, C++, Java).
If you’re such a programmer, don’t worry. There aren’t many Python gotchas. Keep learning Python. It is a great language, and you’ll soon come to love it.
Other posts about Python Gotchas
- Backslashes are escape characters
- Backslashes in Windows filenames
- Forgetting to code the parentheses in method calls
- Specifying mutable default arguments for a function or method
Lists of Python Gotchas
- Steve Ferg’s list of Python Gotchas.
- Hans Nowak’s list of Python Pitfalls.
- Mark Lutz’s instructions on what to do When Pythons Attack.
Python Conquers The Universe 
Another gotcha:
The first is horrid (copied from c), but the combination is super-horrid.
I’m not sure why the above commenter has such an abhorrence for octal notation. It’s in Python because it’s in C, and it’s in C because it’s used all over the place in UNIX (look up ‘octal’ on wikipedia if you’re interested).
Octal notation is only used by Python when the programmer starts a literal number with 0, because there has to be some way to specify an octal literal, and starting with 0 is the tried-and-true notation. If you don’t want your numbers to be in base-8, don’t start your literals with 0.
Octal isn’t used when converting from a string because of filler formatting. e.g. Starting employee IDs with many 0’s because you want to have a fixed ID length, but don’t have 100000 employee’s yet. No one would want those employee IDs to be converted to octal, so that’s not how the int() function works.
In fact, the reason why int(‘010’) returns 10, and not 8, is because the int() function has a ‘base’ parameter which defaults to 10. So
Its like you read my mind! You seem to know a lot
about this, like you wrote the book in it or something. I think that you can
do with some pics to drive the message home a bit, but other than that, this is wonderful blog.
An excellent read. I will certainly be back.